Brian S. Hardy

Peltier-actuated microvalve performance optimization

Valves for microfluidic systems have, for various reasons, proven to be difficult to fabricate, cumbersome to operate, and/or unreliable. We have explored the performance of a novel microfluidic valve formed by creating a flow channel past a Peltier junction. When the Peltier junction is used as a thermoelectric cooler it is possible to freeze the fluid in the valve, forming an ice plug that blocks flow through the valve. This type of valve is fundamentally leak-free, has no moving parts, and is electrically actuated. We have fabricated several experimental prototypes and evaluated their performance. We find that they are reliably capable of closing in less than 100 ms, and of opening substantially faster.

Space Shuttle Foam Debris Monte Carlo Risk Analysis

After the STS-107 Columbia accident, caused by a large piece of Space Shuttle foam debris impacting the reinforced carbon-carbon (RCC) wing leading edge (WLE) of the Orbiter, NASA embarked to “determine critical debris sources, transport mechanisms, and resulting impact areas [and] based on the results of this assessment...recommend changes or redesigns that would reduce the debris risk.” The Aerospace Corporation and NASA developed a Monte Carlo analysis to study the debris impact phenomena in an end-to-end manner. The Aerospace Corporation integrated extensive testing, modeling, and observational data provided by Space Shuttle team members from NASA-JSC, NASA-MSFC, NASA-KSC, NASA-NESC, NASA-Ames, Lockheed Martin, Southwest Research Institute (SwRI), Boeing and The Aerospace Corporation into a simulation-based probabilistic risk assessment of ascent foam debris. This analysis predicts the answers to three questions: what can go wrong (debris release), what and how severe are the potential consequences (debris impact and damage), and what are the probabilities of occurrence for those consequences (resultant risk)? 2

Standoff Pressures and Thermal Characterization of the Peltier-Actuated Microvalve

Experiments were conducted with Peltier-actuated microvalves to determine the maximum stand-off pressures possible with such valves, and to investigate the effect of supercooling on valve operation. Stand-off pressures were found to exceed the limits of measurability at 10 MPa. The experimental data, supported by thermal modeling, indicated that supercooling in linear-junction microvalves was typically about -15 C, while supercooling in tube-type Peltier valves ranged from -8 to -20 C

Aerodynamically Induced Fracture of Ice Shed from the Space Shuttle External Tank

After the STS-107 (Columbia) failure, a major part of the Space Shuttle return-to-flight effort involved risk assessments of Orbiter impact damage due to debris that might be shed by the External Tank (ET) during ascent. One potential source of debris that attracted attention was ice that forms, due to condensation of moisture, on bellows in the liquid oxygen feedline and on brackets holding the line in place. In an effort to gain insight into the fracture characteristics of ice during Space Shuttle ascent, an experiment was designed to investigate the behavior of ice as it accelerates in a supersonic crossflow as well as to study the behavior of ice upon impact with ET insulating foam. These tests demonstrated that ice has good structural resilience to loads associated with aerodynamic acceleration and recontact foam impacts.

Peltier-actuated microvalves: performance characterization

Valves for microfluidic systems have, for various reasons, proven to be difficult to fabricate, cumbersome to operate, and/or unreliable. We have explored the performance of a novel microfluidic valve formed by creating a flow channel past a Peltier junction. Using the Peltier junction as a thermoelectric cooler causes the fluid in the valve to freeze, forming a plug that blocks flow through the valve. Reversing the current in the Peltier junction causes the fluid to melt, reopening the valve. This type of valve is fundamentally leak-free, has no moving parts, and is electrically actuated. We have fabricated an experimental prototype capable of closing in less than one second, and of opening substantially faster. We have also developed a finite-element thermal model of the valve, and exercised it to optimize valve design. An optimized valve is predicted to have a cycle time on the order of 10 ms.

On-orbit characterization of space solar cells on nano-satellites

The Aerospace Corporation has been building, testing, and flying miniature satellites in the pico-and nano-satellite class for over a decade. Significant advances have been made to the bus avionics unit and other satellite subsystems during this time. The advances have enabled various on-orbit tests and experiments, one of which has been to host space solar cell experiment payloads. Recent solar cell flight experiments on Aerospaces CubeSats (AeroCubes) demonstrated several subsystems can simultaneously operate to obtain precise measurements of space solar cell performance. Low cost, rapid return CubeSat missions can be valuable development tools for advancing the readiness level of space technologies.

Along for the Ride: Experience with Flexible Mission Design for CubeSats (Invited)

The AeroCube-4 CubeSat, a picosatellite in a 10-cm cube form factor with a mass of 1 kg, was delivered by an Atlas V launch vehicle into a 480 x 780 km altitude orbit inclined at 65 deg. Despite being deployed into this unconventional orbit, AeroCube-4 has succeeded in performing many missions beyond its original design, including formation control via differential drag and Earth imagery for forest-fire geolocation and sunglint. Under normal circumstances, a mission designer would not select the AeroCube-4 orbit for these activities. The nature of the CubeSat paradigm demands flexibility on the part of the mission designer and on how he or she interacts with the spacecraft designer, working in a community where the orbit may not be known until after the spacecraft is built. The wide-ranging activities of AeroCube-4 are discussed, as are the mission-design challenges that grew out of this satellite’s less-than-optimal orbit.

Combined Environments Testing of Space Shuttle External Tank Foam Defects

Throughout the history of the Space Shuttle program significant amounts of foam debris have been released from the External Tank (ET) during launch posing a significant impact hazard to the Orbiter thermal protection system. Various root causes for this debris were identified and well researched following the Columbia accident, which had resulted from a foam debris impact. One mechanism that can produce debris arises due to the presence of gas-filled voids trapped within the foam. The flight environments can induce the void to vent through the foam surface, allowing the foam above the void to be ejected as debris. Following the Columbia accident, substantial characterization testing was conducted by the ET Project to estimate worst-case mass and release timing for this debris. However, when conducting a probabilistic risk assessment for foam debris, realistic inputs of foam mass and release timing are crucial to producing an accurate risk calculation. For this reason a test was developed and conducted in The Aerospace Corporation’s combined-environments wind tunnel. Samples of ET foam were manufactured with simulated voids at realistic angles of orientation and then exposed to Space Shuttle flight environments. This testing demonstrated a significant reduction in mass and frequency of release for voids that are not fixed in a worst-case orientation relative to the foam surface. This observation is better correlated with photographic evidence acquired after ET separation, which counts the size and number of debris divots of this nature. Further, the time of release was spread out over a longer period of time with a significant portion of debris being expelled at a time of flight when the atmospheric pressure is negligible, which prevents debris acceleration and impact damage.